Fair Value Measurements (Recurring Fair Value Measurements) (Details) (Commodity Derivatives [Member], USD $) In Thousands, unless otherwise specified	Mar. 31, 2012	Dec. 31, 2011
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Financial assets (liabilities)	$ 22,842	$ 33,795
Assets	37,671	44,019
Liabilities	(14,829)	(10,224)
Level 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Financial assets (liabilities)	8,930	180
Assets	23,341	9,698
Liabilities	(14,411)	(9,518)
Level 3 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Financial assets (liabilities)	13,912	33,615
Assets	14,330	34,321
Liabilities	$ (418)	$ (706)